The following changes occur as a country becomes more developed:
Primary activities decrease:
As the country gets richer, it can rely less on its own natural resources and start importing from other countries
Importing products means the country can get a wider range of products at a cheaper price.

Secondary activities increase and then decrease:
It increases as country makes more products but then the country starts importing "ready made products" so less people are needed in the secondary industry.
Machines also take over secondary employment when countries get rich enough to buy them.

Tertiary activities increase:
Machines have taken over employment in other areas so people must seek work in tertiary.
As the country improves its health and education service, more jobs are needed.